wracked or racked?

No, wrack is a seaweed.

To wrack with pain and to wreak havoc have a similar derivation. I would always put a w on the beginning. To me rack is a stack of shelves on which to put shoes etc.

Well I would have put a w, but consensus on the net seems to be no w.

Both spellings are used and accepted eg nerve-wracking, racked my brain, wreak havoc.

Is a wrack an instrument of torture?

No, that's a rack - to stretch the arms and legs of the victim!

Ana that's correct, seaweed - such as bladder wrack, often seen washed up on shore.

But the past tense of "wreak" is surely "wreaked or wrought".
I think the confusion has arisen because of the similarity with "wreck".
It is definitely nerve-racking, racked with pain etc, from as you say, the instrument of torture, the rack.

I would have said 'wracked' but you make sense Alea, so 'racked' it is from now on!

Oxford dictionary says 'wracked with guilt' can be either.

I've gone back to "rack" too, after searching in our dictionary. 7th edition of Concise Oxford dictionary of Current English.
From that it seems that rack means stretch, so rack your brains means stretching your brain to it's limits to find an answer.
"Wrack" is a type of seaweed, as well as being a much older word, derived from the old Norse for wreck. With or without the "k".

Not in my OED, stillhere even the online one fails to find a verb to wrack
wrack is a noun , meaning 1) a type of seaweed, 2) destruction 3) a wreck or wreckage and 4) a rack or parallel formation

I would have said wracked with pain. As the "w" is silent anyway it really does not matter.
But he "wreaked havoc" seems to be being used most if you Google it. "Over wrought" "Wrought iron" are all to do with a stretching process.

I wonder who is using a UK English spelling check and who is using an American one?

Why? Is there a difference in the way it's spelled in the US?

I haven't seen mention of it anywhere...

Ana I don't really know but the Americans do tend to simplify a lot of our UK English spelling.

This link is for those who (can)BA to to find out a bit more.

en.wikipedia.org/wiki/Comparison_of_American_and_British_English

Well, yes, I know they spell certain words differently, Nelliemoser, but wasn't aware that it wasin any way applicable to the 'rack' or 'wrack' issue!

"Wrought" as in " wrought iron" has nothing to do with wreaking, wrecking or racking, it is to do with an archaic usage of "to work, or to do " ("what wonders the Lord hath wrought")

Wrought definition, Archaic except in some senses. a simple past tense and past participle of work
So wrought iron means "worked" iron i.e. rendered decorative by hammering, bending etc.